1. Field of the Invention
The present invention is in the field of 3-D volumetric medical imaging, and, in particular, is directed to an improved system and method to calibrate the images of CT scanners.
2. Description of the Related Art
The original Hounsfield patent, disclosing the first CT scanner in 1973, used water as reference material (Hounsfield, U.S. Pat. No. 3,778,614). All of CT since has been based on water as reference calibration material and lead to the Hounsfield Scale (HU units) in honor of Hounsfield for this most important and Noble Prize winning invention. CT scanners continue to be calibrated to this day with water equivalent phantoms, typically circular solid or liquid water filled cylinders. Such calibrations are essential to CT scanner use in medical imaging and require routine re-calibrations by periodically scanning such phantoms. This requires significant time and effort to determine and continue to maintain scanners within relatively narrow calibration ranges, even for the idealized water phantoms (on the order of +/−3 HU units). The resulting pixel intensities of clinical images are therefore approximately representative of the underlying tissue densities and atomic numbers and provide spectacular images for subjective viewing and diagnosis. However, the pixel values are sufficiently inaccurate and carry significant variability to limit their use in a variety of clinical applications. More accurate and reproducible measurements of the densities of almost all tissues and organ systems of the body have potential clinical value and have long been a goal of the medical imaging industry.
The fundamental limitation of calibration with idealized water cylinders is the inability to accurately simulate the great variability of the human body in terms of size, shape, and composition. The resulting images are not adequately representative of the underlying tissues for individual patients or from scanner to scanner.
These historic limitations of water calibrations have been made much worse with the availability of the more recent multi-detector CT scanners (MDCT) with 64 to 256 detector rows. The increased volume of tissue imaged in each rotation leads to greatly increased scattered radiation at the detectors. One of the advantages of earlier single slice scanners was the resulting avoidance of scatter due to the narrow beam conditions. Indeed the scatter component from single slice scanners was only a few percent of the primary beam. With current MDCT scanners, the scatter can be greater than the primary beam resulting in the requirement that manufacturers use various scatter correction methods to improve the images and remove artifacts. The MDCT images provide much faster scan times allowing cardiac studies and excellent images for viewing. However, the high scatter degrades the MDCT images and leads to greater errors in the HU values. This problem is made even worse by the methods used by the manufacturers to correct for the scatter effects. These corrections, mainly consisting of differing combinations of three methods, are proprietary, differ with manufacturers, create variable and larger errors in HU values, and are not sufficiently specific for individual patients. As a result, the reliability of CT HU values for MDCT scanners has become significantly worse.
There are now several clinical applications that require conversion of the customary HU scale of CT images to density units. This requires additional calibrations with added time and costs. There are therefore advantages to presenting CT images directly in density units instead of the HU units.
Early efforts to measure tissue densities focused on so-called ‘hard tissue’ namely bone. Since bone contains calcium as the primary component, CT scanner HU units were particularly inaccurate because of the increased energy dependence of x-ray attenuation in higher z materials and the variable attenuation in different body regions and bodies of different sizes and compositions. In order to overcome this energy dependence and obtain accurate calibrations, external bone equivalent phantoms have been scanned simultaneously with the patient in bone densitometry (see, for example, U.S. Pat. No. 4,233,507 to Volz; and U.S. Pat. No. 4,922,915 to Arnold). Such phantoms and methods have more recently been applied to calibration of vascular calcifications (see, U.S. Pat. No. 7,558,611 to Arnold). For any tissue which has a significant energy dependent x-ray attenuation in the diagnostic energy range, such calibrations are advantageous and the attenuation properties of the target tissue and the reference calibration phantom must be sufficiently close to allow accurate calibrations. Such methods have been extended to calibration of tissue iodine contrast media density by the similar use of phantoms with iodine reference materials (see, U.S. Pat. No. 8,186,880 to Arnold). In an effort to further improve upon these methods, Arnold disclosed methods that use both external phantoms and internal tissues for a so-called “hybrid” calibration method (see, U.S. Pat. No. 6,990,222). This method has several advantages for calibration of higher atomic number targets and tissues.
A prior art method to calibrate CT scanners without phantoms for bone density measurements was disclosed by Goodenough (see, U.S. Pat. No. 5,068,788). This method uses region-of-interest (ROI) measurements of muscle regions and fat regions to estimate a calibration relationship for bone. The reference regions, namely fat and muscle ROIs, do not have the same energy dependent x-ray attenuations as bone, and required an undisclosed, empirical method to develop a relationship of bone to muscle and fat. The Goodenough and here disclosed methods are similar in that both measure parameters from the histograms of muscle and fat in CT images and compute the density of another tissue. The Goodenough method however requires an operator to manually place the ROIs at specific locations to include fat or muscle in each image to be calibrated and so the method is not automatic. Goodenough did not disclose methods for water calibration of CT scanners or calibration of the displayed images to a density scale.